Optical coherence tomography (OCT) is a noninvasive, noncontact imaging modality that uses coherence gating to obtain high-resolution cross-sectional images of tissue microstructure. In Fourier domain OCT (FD-OCT), the interferometric signal between light from a reference and the back-scattered light from a sample point is recorded in the frequency domain rather than the time domain. After a wavelength calibration, a one-dimensional Fourier transform is taken to obtain an A-line spatial distribution of the object scattering potential. The spectral information discrimination in FD-OCT is accomplished typically by using a dispersive spectrometer in the detection arm in the case of spectral-domain OCT (SD-OCT) or rapidly tuning a swept laser source in the case of swept-source OCT (SS-OCT). Variants of FD-OCT such as Parallel OCT, Full-Field OCT, and Holoscopy have been proposed to overcome limitations and increase the imaging depth.
Many swept sources include a wavelength selectable optical filter, where the optical filter transmission wavelength or frequency is adjusted to cause the laser to sweep in wavelength or frequency. In such designs, the laser cavity length is fixed, limiting the operating wavelengths or frequencies of the swept source to the discrete longitudinal modes of the laser. Thus, as the filter is swept in transmission frequency, the laser will hop from one mode to the next in a discrete manner. The result is that the swept source laser generates a spectral comb function, rather than a smooth distribution of frequencies. As the Fourier transform of a comb function is a comb function, the coherence function of the swept source laser is therefore a comb function. In OCT systems, this leads to an effect known as interference revival or coherence revival (see for example Baek et al., “High-resolution mode-spacing measurement of the blue-violet diode laser using interference of fields created with time delays greater than the coherence time,” Jpn. J. Appl. Phys. 46, 7720-7723, 2007 and Dhalla et al., “Complex conjugate resolved heterodyne swept source optical coherence tomography using coherence revival,” Biomed. Opt. Express 3, 663-649, 2012 hereby incorporated by reference). In OCT systems that do not have coherence revival effect, the interference fringes can only be observed when the optical path lengths of the sample and reference arms are matched (i.e. standard mode zero optical delay position). However, in systems with coherence revival effects, interference fringes can also be observed when the optical path lengths of the sample and reference arms are mismatched by close to an integer multiple of the laser cavity length.
Typical OCT systems have several optical surfaces in the sample arm and it may not be possible to totally eliminate the back-reflections from theses surfaces. Coherence revival poses a serious challenge for optical system design as even small back-reflections from any optical surface in the sample arm that may occur close to an integer multiple of laser cavity lengths away from the intended sample location would result in unwanted interference signal. The resulting interference signals will lead to artifacts superimposed on the actual tomogram. Those artifacts can be quite prominent and may lead to misinterpretation of images or erroneous analysis results. In this document, the terms coherence revival mode, coherence revival artifact, coherence revival signal, coherence revival interference are used interchangeably to refer to interference signal that is achieved when the optical path lengths of the reference arm and the back-scattered signal location are mismatched by close to an integer multiple of laser cavity lengths away from the standard mode zero optical delay position. In this document, we may use the term “pseudo path-matched” in the context of coherence revival interference when the optical path delay between sample (or the location of backscattered signal) and reference arms are mismatched by an integer multiple of cavity lengths.
So far there is to the best of our knowledge no known method for reduction or removal of coherence revival artifacts. One may try to avoid the artifacts by designing the optics of the OCT interferometer such that the location of all optical surfaces or unwanted back-scattering locations do not overlap with the positions of the optical path where coherence revival occurs (i.e. close to an integer multiple of laser cavity length away from the standard zero delay position). In complex optical systems this may however not always be possible.
Dhalla et al. showed that some commercially available external cavity tunable lasers (ECTLs) exhibit a relative spectral frequency shift between light of the sample arm and light of the reference arm, when interference happens in the coherence revival mode (i.e. reference arm and sample arm are offset by an integer multiple of the laser cavity length) (see for example Dhalla et al., “Complex conjugate resolved heterodyne swept source optical coherence tomography using coherence revival,” Biomed. Opt. Express 3, 663-649, 2012 hereby incorporated by reference). The phase modulation results in a frequency shifted interferogram in the coherence revival mode. In normal mode (i.e. if the optical path lengths of reference arm and sample arm are closely matched), the phase modulation is not detectable by OCT, since the phase modulation effect is identical in the reference and sample beams. It is believed that the phase modulation in these ECTLs is caused by the SOA inside the laser cavity.
Dhalla et al. showed that, similar to the heterodyne detection in SS-OCT by use of electro optic or acousto optic modulators in one of the arms (see for example Yun et al., “Removing the depth-degeneracy in optical frequency domain imaging with frequency shifting,” Opt. Express 12, 4822-4828, 2004), the frequency shifted interferogram in coherence revival mode may be used for complex conjugate resolved heterodyne SS-OCT. In this method, a carrier frequency is effectively introduced to the original spectral fringe signal to resolve the complex conjugate signal. Yun et al. introduced the relative frequency shift between reference and sample beam using acousto optic modulators in reference and sample arm. Dhalla et al. didn't require additional frequency shifters, because the SOA inside the cavity created a similar relative frequency shift in coherence revival mode. In standard OCT systems, typically zero frequency fringes in the interference spectra correspond to zero optical path delay. However in complex conjugate resolved heterodyne SS-OCT, the true zero optical path delay results in spectral fringes at the carrier frequency. Hence, the formerly positive and negative frequencies are all shifted towards one side of zero frequency while the best fringe visibility is maintained at the depth corresponding to matched group delay. The shift caused by the carrier frequency allows for distinction between positive and negative frequencies and also helps in making use of the full coherence length. However, as one still performs the Fourier transform of a real valued signal, the signal after the Fourier transform remains Hermitian.
In addition to the work by Yun et al, Zhang et al. published results using an electro-optic phase modulator (EOM) in the reference arm to create a similar frequency shift of the fringe signal (Zhang et al., “Removal of a minor image and enhancement of the signal-to-noise ratio in Fourier-domain optical coherence tomography using an electro-optic phase modulator,” Opt. Lett. 30, 147-149, 2005). Davis et al. reported a system using acousto optic frequency shifters in the reference arm and sample arm (Davis et al., “Heterodyne swept-source optical coherence tomography for complete complex conjugate ambiguity removal,” J. Biomed. Opt.10, 064005, 2005). None of these groups explored the possibility of reducing coherence revival artifacts, but rather focused on use of the coherence revival mode for complex conjugate resolved SS-OCT instead of the standard coherence mode for imaging. Dhalla et al. makes the suggestion that the laser may be designed to produce an optimized coherence revival mode for complex conjugate resolved heterodyne SS-OCT, but makes no suggestions regarding how to minimize the coherence revival mode signal or how these designs may be achieved.